Fibrous web and process for the preparation thereof

ABSTRACT

The invention relates to a fiber web and a method for preparing it. The fiber web comprises a filler-containing base web, which is possibly coated with a pigment-containing coating layer. According to the invention, 5-100% of the filler in the base web is made up of cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon. These coated cellulose fibrils or lignocellulose fibrils constitute at maximum approx. 70% of the weight of the base web. The base web according to the invention contains more than 20% less retention agents than does a web which has the same formation but in which more than 95% of the filler is made up of filler particles known per se.

This application is a continuation of application Ser. No. 10/475,771, which is a 371 of PCT/FI02/00344 filed Apr. 24, 2002, which claims priority of Finnish patent application 20010847 filed Apr. 24, 2001, each of which is incorporated herein by reference.

The present invention relates to the fiber web comprising a filler-containing base web possibly coated with a pigment-containing coating layer.

A fiber web such as this in general comprises a filler-containing base web possibly coated with a pigment-containing coating layer.

The invention also relates to the method for the preparation of a fiber web in which there is prepared from a fiber material a slush from which a fiber web is formed in a paper or board machine.

The slush in papermaking is made up of cellulose fibers or lignocellulose fibers, fines and fillers. Many of these components, in particular fillers, are so small in size that they will not adhere mechanically to the wire and they have to be bound to larger fibers or to each other by flocculation. For flocculation there are used retention chemicals which are added to the slush before the forming of the web. Retention chemicals to be mentioned include polymeric products such as polyethylene imine, low molar mass polyacrylamide and polyamine, as well as cationic starch, guar or polyacrylamine combined with colloidal silica, alumina or montmorillonite. The amount of retention chemicals is in general at least 0.5% of the dry matter of the fiber, typically approx. 0.6-1% of the dry matter of the fiber.

In papermaking, the evenness of the web, i.e. formation, and the remaining of slush components on the wire, i.e. retention, are normally always compromises of some degree; when one of them is improved, the other one deteriorates. This is logical, since retention normally requires the flocculation of small particles into clusters, whereupon the evenness (i.e. formation) of paper deteriorates. For this reason it is often necessary to run the paper with a lower than desired retention in order to produce even paper, which increases the amount of solids in the circulation waters and generates various problems. Furthermore, the retention systems at present available commercially are very sensitive to chemical changes in circulation water.

It is an object of the present invention to eliminate the disadvantages associated with the state of the art and to provide a totally novel solution for producing a fiber web that has simultaneously both good retention and good formation.

The invention is based on the observation that the amount of conventional retention agents can be reduced significantly by replacing at least a portion of the conventional mineral fillers, i.e. pulverous mineral pigments, with a composite pigment containing light-scattering material particles deposited on the surface of the fines. A filler product of this type is known from FI patent publication 100729. The product comprises strings of pearls made up of calcium carbonate particles deposited on the fines and of fine fibrils. According to the patent publication, it is characteristic of this filler of a novel type that calcium carbonate is deposited on fine fibrils prepared from cellulose fibers and/or mechanical pulp fibers by beating. The size distribution of the fines fraction mainly corresponds to wire screen fraction P100.

On the basis of the said patent publication, by means of the filler it is possible to increase the concentration of calcium carbonate in paper, whereby the grammage of the paper can be reduced without changing the “other important” properties of the paper. The filler retention is good. The results of the publication are based on results measured from laboratory sheets by standards SCAN-C 26:76 and respectively SCAN-M 5:76. In the laboratory sheets, cationic starch was used in an amount of 0.65% and silica in an amount of 0.15% of the mass of the fiber.

In connection with the present invention it has been observed unexpectedly that with the composite fillers described above there is obtained at a high level of formation so high a fiber slush retention in the paper or board machine that conventional retention agents are not required at all, or their amount is considerably lower than in conventional base webs.

According to the invention, 10-100% of the filler in the base web is made up of cellulose fibrils or lignocellulose fibrils on which there are deposited light-scattering material particles, and these coated cellulose or lignocellulose fibrils constitute at maximum approx. 70% of the weight of the base web.

More precisely, the fiber web according to the invention is characterized in that 5-100% of the filler in the base web is made up of cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon and the coated cellulose fibrils or lignocellulose fibrils constitute at maximum approx. 70% of the weight of the base web.

The method according to the invention for the preparation of a fiber web is characterized in that to the slush of fiber material there is added as a filler a product comprising cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon, this product constituting 5-100% of the filler and its amount being at maximum 70% of the mass of the fiber material, and retention agents are dosed into the slush in an amount of 0-0.40% by weight of the mass of the fiber material.

The invention provides considerable advantages. Thus, by means of the invention, paper of uniform quality as well as a high level of formation and high retention can be produced; additionally the circulation waters are substantially cleaner and the retention agent requirement is reduced. The invention does not require any flocculation of the filler, contrary to the commercially available retention systems; this has a fundamental effect on formation and the optical effect of the filler.

Better formation leads to smoother and glossier paper. If the paper is coated, the cover provided by the coating is better, which makes smaller amounts of coating possible. Problems associated with the unevenness of gloss and print quality are reduced.

The base web does not contain any conventional retention agents, or their amount is over 20% smaller, preferably up to 50% smaller, than that of a web that has the same level of formation and contains conventional particulate fillers. The option according to the invention therefore decreases the chemicals costs in the preparation of a fiber web and facilitates the recycling of waters in a paper or board machine. According to the invention it is possible to decrease the amounts of both conventionally used nanoparticles and polymers (cf. above), or either one of them. The said reduction of 20% can thus be calculated separately from one of the components or from both components in total. In Example 3 below, the amount of the polymer component has been reduced by slightly less than 30% by weight while the quantity of nanoparticles has remained unchanged.

The invention will be described below in greater detail with the help of a detailed specification and a few embodiment examples.

FIG. 1 shows, of the results of Example 2, the web formation as a function of the total retention, and FIG. 2 shows the formation as a function of the wire water consistency.

In the examples below, the filler used contains a fines fraction comprising fibrils obtained from a chemical pulp. By “chemical pulp” is meant in this context a pulp that has been treated with digestion chemicals for the delignification of cellulose fibers. According to one preferred embodiment, the fibrils used in the invention are fibrils obtained by beating from pulps prepared by the sulfate process and by other alkaline processes. In addition to chemical pulps, the invention is also suited for fillers produced from fibrils obtained from chemimechanical and mechanical pulps.

Typically the average thickness of cellulose or lignoellulose fibrils is smaller than 1 μm. The fibrils are characterized by one or both of the following criteria:

-   -   a. they correspond to a fraction which passes a 50 mesh screen;     -   b. their average thickness is 0.01-10 μm (most suitably at         maximum 5 μm) and their average length is 10-1500 μm.

The source material for the fibrils, i.e. the fines based on cellulose or other fibers, is fibrillated by beating in a pulp refiner. The desired fraction may, when necessary, be separated using a screen, but the fines need not always be screened. Suitable fibril fractions include wire screen fractions P50-P400. Preferably refiners with grooved blades are used.

The light-scattering material particles in the filler are inorganic or organic salts which can be formed from their source materials by precipitation in an aqueous medium. Such compounds include calcium carbonate, calcium oxalate, calcium sulfate, barium sulfate, and mixtures thereof. The material particles are precipitated on the fibrils in order to produce an aggregate resembling a string-of-pearls. The amount of the inorganic salt compound is approx. 0.0001-95% by weight, preferably approx. 0.1-90% by weight, most suitably approx. 60-80% by weight, calculated from the amount of filler, and approx. 0.1-60% by weight, preferably approx. 0.5-50% by weight of the paper.

The invention is discussed below by using the product according to FI patent publication 100729 as an example, but it is clear that it is possible to use in the invention any other of the above-mentioned products which contain various light-scattering pigments.

The filler is prepared by depositing the mineral pigment on the surface of fine fibrils pre-pared from cellulose fibers and/or mechanical pulp fibers. For example, the precipitation of calcium carbonate can be carried out by feeding into an aqueous slush of fibrils an aqueous calcium hydroxide mixture, which possibly contains solid calcium hydroxide, and a compound that contains carbonate ions and is at least partly dissolved in water. It is also possible to introduce carbon dioxide gas into the aqueous phase, which gas in the presence of calcium hydroxide produces calcium carbonate. There form string-of-pearls-like calcium carbonate crystal aggregates which are held together by fibrils, i.e. fine strands, and in which the calcium carbonate particles are deposited onto the fine fibrils and attached to them. The fine fibrils together with calcium carbonate form string-of-pearls-like strands, which primarily resemble strings of pearls in a pile. In water (slush) the ratio of the effective volume of the aggregates to the pulp is very high compared with the corresponding ratio of conventional calcium carbonate used as filler. By “effective volume” is meant the volume required by the pigment.

The diameter of the calcium carbonate particles in the aggregates is approx. 0.1-5 μm, typically approx. 0.2-3 μm. Usually fibrils corresponding in the main (at least more than 55%) to wire screen fractions P50-P400 are used.

The paper pulp is slushed in a manner known per se to a suitable consistency (typically a solids content of approx. 0.1-1%) and is spread onto the wire. There is added to the fiber slush, most suitably in the headbox of the paper or board machine, the above-mentioned filler, usually in an amount of approx. 1-100% by weight of the weight of the fibers in the fiber pulp, in other words the amount of filler may be up to equal to the amount of the actual fiber pulp. Usually the disclosed filler constitutes at least 5% by weight, most suitably 10-100% by weight, of the filler in the base web, and respectively 10-50% by weight of the fiber material in the base web. It is in principle also possible to prepare a base web in which the fiber material in its entirety is made up of filler fibrils, and thus in general the present filler may constitute 1-100% by weight of the fiber material in the base web. Preferably coated cellulose fibrils or lignocellulose fibrils constitute at maximum approx. 70%, e.g. approx. 10-65%, of the weight of the base web, in which case the rest of the web comprises conventional mechanical and/or chemical pulp used in papermaking and/or conventional fillers and/or other additives.

A portion (at maximum 95%, usually 90-10% by weight, of the total amount of filler) of the filler used in the slush may consist of conventional fillers, such as calcium carbonate (natural or precipitated), kaolin, talc, hydrogenated aluminum oxides (aluminum trihydroxides), calcium sulfate, barium sulfate, calcium oxalate, or titanium dioxide. Preferably, however, at least 80%, especially preferably at least 90%, of the precipitated light-scattering pigment particles are attached to fibrils.

By “conventional filler” is meant here a particulate filler which is pulverous and comprises loose particles, whereas in the present invention the filler used is mainly a product in which the particles are not loose but attached to fibrils. According to one viewpoint of the invention, the formation and retention of paper are improved by using a portion of the filler in a form bound to fibrils. Thus, according to this embodiment, the filler used is at least partly for example calcium carbonate, calcium oxalate, calcium sulfate or barium sulfate, of which a portion is in a pulverous form and a portion in a form attached to fibrils. In this embodiment, in order to improve formation, a portion (at least approx. 5% by weight, preferably at least 10% by weight, most suitably at least 20% by weight) of the pulverous filler is replaced with a product attached to fibrils.

In the paper or board machine the fiber pulp is formed into a paper or board web. The fiber web is dried and most suitably coated, and optionally after-treated by for example calendering.

The web can be coated with, for example, calcium carbonate, gypsum, aluminum silicate, kaolin, aluminum hydroxide, magnesium silicate, talc, titanium dioxide, barium sulfate, zinc oxide, synthetic pigment, or mixtures thereof.

The light-scattering material particles of the filler in the web are present in the fiber web in a substantially unflocculated form. This means that the formation of the web is quite good. Thus, at formation level 10 the web contains conventional retention agents, such as cationic starch and/or silica, in total at maximum 0.40% by weight of the mass of the fiber. According to an especially preferred embodiment the web is substantially or entirely free of ionic retention agents. “Conventional retention agents” in particular include those used together with conventional fillers.

With the help of the invention it is possible to produce coated and optionally also calendered cellulose-containing material webs having excellent printing properties, high smoothness, as well as high opacity and whiteness. By “cellulose-containing material” is meant here generally paper or board or a corresponding cellulose-containing material derived from a lignocellulose-containing raw material, in particular wood or annual or perennial plants. The said material may be wood-containing or woodfree, and it may be prepared from mechanical, semimechanical (chemimechanical) or chemical pulp. The chemical pulp and the mechanical pulp may be bleached or unbleached. The material may also contain recycled fibers, in particular recycled paper or recycled board. The grammage of the material web varies typically within the range 35-500 g/m², in particular it is approx. 50-450 g/m².

In general the grammage of base paper is 20-250 g/m², preferably 30-80 g/m². By coating a base paper of this type, having a grammage of approx. 50-70 g/m², with 10-20 g of coating/m²/side and by calendering the paper, there is obtained a product having a grammage of 70-110 g/m², whiteness of at least 90% and opacity of at least 90%. An especially preferred product is a coated offset paper in which high gloss and high opacity and bulk are combined. The invention is also suited for the production of coated fine papers, possibly also containing mechanical pulp, as well as writing and printing papers.

The following non-restrictive examples illustrate the invention. The measuring results indicated in the examples for the properties of the paper were determined by the following standard methods:

Surface roughness: SCAN-P76:95

Porosity: SCAN-P60

Air resistance: SCAN-M8, P19

EXAMPLE 1 Preparation of Filler

Beating of Chemical Pulp

Birch sulfate pulp was beaten in Valmet's JC-01 refiner in order to produce a pulp suitable for the preparation of filler. The consistency during the beating was approx. 4%, its total energy consumption 343 kWh/t and its specific edge load 0.5 J/m.

The properties of the product are shown in Table 1. TABLE 1 Fiber properties before and after beating Before beating After beating Fiber length (length), mm 0.86 0.58 Fiber length (weight), mm 1.00 0.77 SR° 16 86 Carbonation of Fiber Pulp

Carbonation was carried out in tap water in accordance with what is disclosed in FI patent 100729. An aqueous slurry having a dry matter content of 2.22% was obtained. The concentration of CaCO₃ in the final product was 69.7% and its specific surface area was 10.6 m²/g. The PCC particle size was of an order of magnitude corresponding to that in Example 1 of the FI patent.

EXAMPLE 2

The product described in Example 1 was used as filler in coated fine papers. The following table depicts the results of a fine paper test run carried out in Stockholm with STFI's pilot machine (FEX): TABLE 2 Results of fine paper runs Filler concentration, Wire water Total Formation, % consistency, g/l retention, % % PCC 18.8 2.03 72.9 10.6 PCC 21.0 2.68 66.9 11.1 SuperFill 17.8 0.48 91.1 10.4 SuperFill 22.4 0.67 88.3 9.5

The PCC used in the tests was Albacar LO product supplied by Specialty Minerals.

The advantage of the invention is well visible when the properties are examined simultaneously in the same coordinate system in the manner shown in accompanying FIGS. 1 and 2.

The method is provided special value by the fact that the above-mentioned combinations of properties are achieved simultaneously with better optical properties and strengths.

EXAMPLE 3 Mill Trial

Preparation of Filler-Containing Base Paper

For purposes of coating, base paper having a grammage of 56 g/m² was prepared in mill conditions. The slush consisted of a mixture of birch pulp (74%) and pine pulp (24%). After beating, the SR number of the pine pulp was 32-34° and that of the birch pulp was 22-25°. The SR number of the pulp in the head box was 35-40°.

The wire section of the paper machine was Valmet's hybride wire (Sym-former), and the wet press end comprised Valmet's Sym-Press II having a triple press and a conventional drying section.

Three different fillers were used in the base paper, namely Finntalc F 15 SL (talc of Mondo Minerals), Albacar HO (PCC of Specialty Minerals), and the composite filler described in Example 1, of which the name “SuperFill” is also used hereinafter. Talc was used as filler in amounts of 10% and 15%, and PCC and SuperFill in amounts of 10%, 15% and 20%.

The retention chemicals used were nanoparticles and cationic starch (Compozil Plus: EKA NP 780 nanoparticle and EKA PL 1510 C-Pam, supplier: EKA Chemicals). For talc and PCC the doses were as follows: nanoparticle 280 g/t, polymer 70 g/t, and those for SuperFill were: nanoparticle 280 g/t, polymer 50 g/t. The amounts of cationic starch and resin size used were 8 kg/t and 5.2 kg/t. Alum was dosed for talc in an amount of 13 kg/t and for PCC and SuperFill in an amount of 19 kg/t.

Results

The different fillers showed no notable differences in circulation water conductivity, COD, pH, cation requirement, or dissolved calcium concentrations. Filler retention was best with SuperFill (40-50%) even though the dose of retention polymers was considerably smaller than with other fillers. The filler retention with talc and PCC was only 30-40%. The good retention of SuperFill significantly reduces the consistency and turbidity of circulation water as compared with the other fillers. No differences were observable in beta formation among the different filler and filler concentrations. 

1. A fiber web comprising a filler-containing base web optionally coated with a pigment-containing coating layer, characterized in that 5-100% of the filler in the base web is made up of cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon, and the coated cellulose fibrils or lignocellulose fibrils constitute at maximum approx. 70% of the weight of the base web, wherein the base web contains 0 to 0.40% by weight of the mass of the fiber of a conventional retention agent selected from cationic starch and silica at a beta-formation of
 10. 2. The fiber web according to claim 1, characterized in that the filler comprises cellulose fibrils or lignocellulose fibrils prepared from vegetable fibers by beating and screening and having an average thickness smaller than 10 μm.
 3. The fiber web according to claim 2, characterized in that the light-scattering material particles are deposited on fibrils which correspond to a fraction passing a 50 mesh screen and/or the average thickness of which is 0.01-5 μm and average length 10-1500 μm.
 4. The fiber web according to claim 1, characterized in that the light-scattering material particles are inorganic or organic salts which can be formed from their source materials by precipitation in an aqueous medium.
 5. The fiber web according to claim 4, characterized in that the light-scattering material particles are calcium carbonate, calcium oxalate, calcium sulfate, barium sulfate or a mixture thereof.
 6. The fiber web according to claim 1, characterized in that the material particles are deposited on fibrils in order to produce a string-of-pearls-like filler.
 7. The fiber web according to claim 6, characterized in that the material particles are a calcium compound selected from the group consisting of calcium carbonate, calcium oxalate, and calcium sulfate and the amount of the calcium compound in proportion to the fiber amount is 0.0001-90% by weight.
 8. The fiber web according to claim 1, characterized in that it is coated with calcium carbonate, gypsum, aluminum silicate, kaolin, aluminum hydroxide, magnesium silicate, talc, titanium dioxide, barium sulfate, zinc oxide, synthetic pigment, or a mixture thereof.
 9. The fiber web according to claim 1, characterized in that the filler comprising cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon constitutes 10-100% by weight of the filler in the base web.
 10. The fiber web according to claim 1, characterized in that the filler comprising cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon constitutes 10-50% by weight of the fiber material in the base web.
 11. The fiber web according to claim 1, characterized in that the light-scattering material particles in the filler of the web are present in a substantially unflocculated form in the fiber web.
 12. The fiber web according to claim 1, characterized in that it is substantially free of ionic retention agents.
 13. The fiber web according to claim 1, characterized in that it comprises coated fine paper or printing or writing paper.
 14. The fiber web according to claim 1, characterized in that 5-90% of the filler in the base web is made up of cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon.
 15. The fiber web according to claim 14, characterized in that the balance of the filler is made up of conventional pulverous fillers.
 16. A method for preparing a fiber web, according to which method there is prepared from a fiber material a slush from which a fiber web is formed in a paper or board machine, characterized in that to the slush of fiber material there is added as a filler a product comprising cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon, this product constituting 5-100% of the filler and its amount being at maximum 70% of the mass of the fiber material, and retention agents selected from cationic starch and silica are dosed into the slush in an amount of 0-0.40% by weight of the mass of the fiber material.
 17. The method according to claim 16, characterized in that a filler is used which comprises cellulose fibrils or lignocellulose fibrils prepared from vegetable fibers by beating and screening and having an average thickness smaller than 10 μm.
 18. The method according to claim 17, characterized in that a filler is used wherein the light-scattering material particles are deposited on fibrils which correspond to a fraction passing a 50 mesh screen and/or the average thickness of which is 0.1-5 μm and average length 10-1500 μm.
 19. The method according to claim 16, characterized in that a filler is used wherein at minimum 90% of the precipitated light-scattering pigment particles are attached to fibrils.
 20. The method according to claim 16, characterized in that the base web is coated in the paper machine with a pigment-containing coating mix.
 21. The fiber web according to claim 2, characterized in that the filler comprises cellulose fibrils or lignocellulose fibrils prepared from vegetable fibers by beating and screening and having a maximum average thickness of 5 μm.
 22. The method according to claim 16, characterized in that a filler is used which comprises cellulose fibrils or lignocellulose fibrils prepared from vegetable fibers by beating and screening and having a maximum average thickness of 5 μm.
 23. The method according to claim 16, characterized in that no ionic retention agents are dosed into the slush.
 24. The fiber web according to claim 16, characterized in that the conventional pulverous fillers are selected from natural or precipitated calcium carbonate, kaolin, talc, hydrogenated aluminum oxides, calcium sulfate, barium sulfate, calcium oxalate or titanium dioxide 